4.7.7 - Respiratory substrates and quotient

• Respiratory Substrates and Quotients

  • A respiratory substrate is a molecule from which energy can be released in order to synthesise ATP
  • Glucose is the most common substrate for respiration
  • Carbohydrates, lipids and proteins can all be used as respiratory substrates
• Different respiratory substrates

  • Some cells, such as liver cells, often use lipids and proteins as reactants for respiration
  • Carbon atoms from lipids are detached to form acetyl CoA and fed directly into the Krebs Cycle
  • Proteins are broken into amino acids, which are deaminated to remove nitrogen before being converted to pyruvate or acetyl CoA
• Energy values of different substrates

  • The energy value of a substrate is related to how many hydrogens there are in the molecule
  • Hydrogen becomes oxidised and forms water
  • Lipids have more hydrogen by mass than both proteins and carbohydrates and so have higher energy values:
  • Lipids - 39KJ/g
  • Carbohydrates - 16KJ/g
  • Proteins - 17KJ/g
• Respiratory Quotient

  • Respiratory Quotient is the ratio of the amount of CO2 produced to O2 consumed
  • CO2 consumption/O2 consumption
  • Can be used to identify the respiratory substrates an organism uses when undergoing aerobic respiration
  • Carbohydrates have a respiratory quotient of 1
  • Lipids have a respiratory quotient of 0.7
  • Proteins have a respiratory quotient of 0.9